1. Field of the Invention
This invention relates to a keyboard apparatus for an acoustic piano having key sensor switches incorporated therein for generating key depression information, such as a muting piano and an autoplay piano.
2. Prior Art
The muting piano is a piano which is capable of giving not only a normal acoustic performance by causing hammers to strike strings e.g. when the performance is given during daytime, but also a muting performance by causing a built-in tone generator to generate electronic musical tones based on key depression information for a player to listen to the electronic musical tones through headphones, while preventing hammers from striking the strings, thereby preventing the musical tones from bothering neighbors. In this case, the key sensor switches are used for generating key depression information during the muting performance for generating the electronic musical tones.
The autoplay piano is a piano adapted to give an automatic performance by driving under control of a control unit an actuator, such as a solenoid, which is provided under each key at a rear end portion thereof, at predetermined timings, to cause the actuator to thrust the rear end portion of the key upward. In this case, the key sensor switches are used for generating key depression information during the acoustic performance by the autoplay piano to store the key depression information into a memory. The control unit directs the automatic performance by reading out the key depression information stored in the memory.
A key sensor switch employed in these kinds of acoustic piano has been conventionally proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 3-161795, which includes a shutter plate fixed to the bottom surface of the associated key, and two photoswitches arranged below the shutter plate at respective upper and lower levels. Each photoswitch is formed by a photocoupler comprised of a light-emitting element, such as a light-emitting diode, and a light-receiving element, such as a photodiode. When a key is depressed, the shutter plate associated therewith passes between the light-emitting element and the light-receiving element of each photoswitch to block of light transmitted from the light-emitting element toward the light-receiving element. The light-blocked state is detected by the light-receiving element to generate key depression information, i.e. occurrence of a key depression and the speed of the key depression (velocity). Thus, the key depression information is generated in the non-contacting state of keys in which the keys are not brought into mechanical contact with the key sensor switches, thereby preventing the key touch from being adversely affected during the acoustic performance.
However, in the case of the proposed key sensor switch, two photoswitches formed of expensive photocouplers are necessitated for each key, and hence the component cost of the key sensor switches becomes very high. Further, the key sensor switch is constructed such that it generates key depression information when the shutter plate blocks off light transmitted from ore element to the other of each of the photoswitches arranged at different levels. Therefore, it is necessary to mount each shutter plate with a very high accuracy. Therefore, the position adjustment of a shutter plate becomes a very delicate and troublesome matter, which, if carried out on all of the eight-eight keys of the keyboard, necessitates an immense tine period, resulting in a large increase in the manufacturing cost.